Simple answer:

Microsoft brands Windows releases by a name ("Windows Server 2008 R2"). The internal version number (6.1) and build number (7601) have not been used in marketing since Windows NT 4.

Canonical brands Ubuntu releases by both a name ("Lucid Lynx") and the date of release (10.04).

Apple brands Mac OS X releases by both a name ("OS X Lion") and a version number (10.7.2).

Just as the Ubuntu names are either synonyms of their version numbers ([tag:ubuntu-hardy] to [tag:ubuntu-8.04]) or non-existent ([tag:ubuntu-lucid] vs. [tag:ubuntu-10.04]), so should the Mac OS X names either be synonymed or blackholed.